Otherwise operator

XPath 4.0 proposes an additional operator: otherwise. The expression A otherwise B returns the value of A unless it is an empty sequence, in which case it returns the value of B. For example, the expression:

returns the chapter(s) entited "Introduction" if such a chapter exists; otherwise it returns the first chapter.

As defined in the current XPath 4.0 draft (September 2024), arithmetic operators such as + bind more tightly than otherwise, so $a + $b otherwise $c means ($a + $b) otherwise $c. However, Saxon 12 implements an earlier draft of the specification in which it means $a + ($b otherwise $c). Use parentheses to make your intended meaning clear and to avoid compatibility problems in the future.